The present invention relates to a multiband antenna matching unit and, more particularly, to a multiband antenna matching unit comprising multiple transformers, at least one of which can be bypassed to efficiently shift the operating frequency of the matching unit while maintaining the same transformation ratio.
In many environments there is a need to couple an unbalanced transmission line (for example, a 50.OMEGA. coaxial cable) to a balanced line, such as a dipole antenna. Conventional arrangements rely on a balun or a fixed transformer with selectable taps to transform an input unbalance load to a balanced output load. In order to operate over a 10:1 (or larger) frequency range (for example, 3-30 Mhz), a conventional fixed inductor and variable capacitor is insufficient (such an arrangement is capable of covering a frequency range of only about 3:1). To achieve the desired 10:1 range, one typically changes the value of the inductor, by methods such as by selecting taps on a coil, varying an expensive roller inductor, or physically (or electrically) changing the inductor.
When using the tap selection arrangement, corona discharge must be avoided by shorting the unused turns, which results in increasing the loss in the inductor. An alternative exemplary prior art solution to this problem is illustrated in FIG. 1, which is defined as a "Z match" antenna matching unit 10. In this arrangement, the inductor includes a pair of output links 12, 14 to switch between a low impedance load (link 12) and a high impedance load (link 14). The variable capacitance on the unbalanced input includes a first capacitance 16 and second capacitance 18, where second capacitance 18 is added (i.e., paralleled) in lower operating frequency ranges. As shown, "Z match" antenna matching unit 10 s capable of providing tuning over a range of approximately 3.5-30 Mhz by electrically "switching" between the tank circuits formed by inductor 15 and capacitors 16 and 18 (see the article entitled "The Z-Match Coupler--Revisited and Revised" by C. A. Lofgren, appearing in The ARRL Antenna Compendium, Vol. 3,1992, page 191 et seq. for a complete discussion, the Lofgren article being herein incorporated by reference). A problem remaining with this arrangement is the presence of circulating currents, as indicated by the arrows, through the non-resonant variable capacitor and inductor combination, thereby significantly reducing the efficiency of the matching arrangement.
Thus, a need remains in the prior art for an antenna matching arrangement that is capable of operating over a relatively large range (10:1, for example) without suffering from the reduction in efficiency present in the prior art.